The present invention relates to a method and a device for contacting a surface point on a workpiece, in particular for determining a spatial coordinate of the surface point. More particularly, the invention relates to the field of coordinate measurement technology, i.e. the one-, two- or three-dimensional measurement of workpieces by means of measurement tools which make it possible to determine spatial coordinates of the workpieces at selected measurement points. A typical coordinate measuring machine has a measuring head which can be moved relative to the workpiece. The measuring head carries a sensor, by means of which the measuring head can be brought into a defined position with respect to a surface point on the workpiece. Often, the surface point is contacted or touched by means of a stylus. Accordingly, the measuring head in such coordinate measuring machines is typically referred to as a contact probe head. The spatial coordinates of the touched surface point can be determined after the contact from the position of the probe head in the measurement volume and, when possible, from the displacement and deformation of the stylus. By contacting a plurality of surface points on a workpiece, it is possible to record geometrical dimensions and shape profiles.
However, the invention is not restricted to coordinate measuring machines in the narrow sense. It may also be used for machine tools or other machines in which a surface point on a workpiece is contacted by means of a stylus, whether for measurement purposes or for other reasons.
For applications requiring a high measurement accuracy, it is necessary to also take into account elastic deformations of the stylus and even of the coordinate measuring machine during the process of contacting. For this purpose, the contacting force, i.e. the force with which the stylus presses on the surface point, need to be known or determined. On the one hand, a small contacting force is desirable so that the elastic deformations on the stylus, the coordinate measuring machine and also the workpiece can be kept small. On the other hand, a certain contacting force is necessary in order to ensure that the stylus touches the selected surface point “correctly”. An exactly defined contacting force is also desirable for measuring or contacting highly flexible workpieces, which yield in response to the contacting movement.
Persons skilled in this field know that the defined rest position of a mobile stylus on the probe head base depends up to a certain extent on the prior deflections of the stylus. This is the result of various effects or influences such as friction, magnetic remanence or material stresses. Overall, the stylus has a hysteresis behavior which, although it may be minor, nevertheless has an effect in the case of high measurement accuracies.
Various proposals have been made in order to minimize the effects of the hysteresis behavior. DE 10 2004 007 968 A1 proposes to contact the surface point of the workpiece several times in succession, with different alternating contacting forces being used. In this context, it is disclosed that the setting of the contacting forces is likewise subject to a certain hysteresis. Nevertheless, the proposed method allows to increase the measurement accuracy by averaging the acquired measurement values. A disadvantage of this procedure is however that the surface point has to be contacted several times with varying contacting forces, which increases the measurement time on the one hand and is difficult in the case of highly flexible workpieces on the other hand.
There are a plurality of other proposals for minimizing hysteresis effects by proper mechanical construction of the probe head. Reference is made to DE 35 29 320 C2, DE 40 27 136 A1, U.S. Pat. No. 5,253,428, JP 08-304059 and JP 2005-055345 for example.
DE 102 29 824 A1 proposes a method for operating a coordinate measuring machine which can be used in at least two different operating modes. In a first operating mode the probe head is moved by CNC control, i.e. automatically. In a second operating mode the probe head is moved manually, with an operator operating a control console or the like. It is proposed that contacting of the surface point should be carried out in both operating modes with the same directionality, in order to avoid different measurement results due to different hysteresis effects.
JP 9-141815 discloses to determine control signals for an engraving tool as a function of image data which are acquired from previously engraved positions in order to reduce inaccuracies or errors in the engraving process due to hysteresis effects.